1. Introduction
This invention relates to removal of contaminants from organic solutions. More particularly, this invention relates to removal of ionic and organic contaminants from organic solutions immediately prior to their use. The invention is especially useful for the removal of contaminants from solutions used in integrated circuit manufacture.
2. Description of the Prior Art
Ultra pure liquids free of particulate, ionic and organic contamination are required for many industrial purposes such as for the manufacture of pharmaceuticals and integrated circuits. For example, in the manufacture of high resolution integrated circuits, it is known that many processing liquids come into contact with a bare wafer or a resist-coated surface. These include photoresists and treatment chemicals such as organic liquids and aqueous solutions which contain acids, bases, oxidants, and other proprietary ingredients. At least 15 to 50 liquids of various compositions are used to clean wafers, prime surfaces, deposit resists or other polymers, develop, rinse, etch, and strip the resist. It is known that these solutions may be a source of contamination of the integrated circuit wafer that can interfere with its performance. Thus, the reduction or removal of insoluble and soluble contaminants from processing fluids used for the production of integrated circuits before or during use is basic insurance for prevention of damage to the integrated circuit.
Photoresist coating compositions are used extensively in integrated circuit manufacture. Such compositions typically comprise a light-sensitive component and a polymer binder dissolved in a solvent. Typical photoresist compositions are disclosed in U.S. Pat. Nos. 5,178,986; 5,212,046; 5,216,111; and 5,238,776, each incorporated herein by reference for disclosure of photoresist compositions, processing, and use. For example, as disclosed in said patents, a typical photoresist composition would be coated onto a substrate, dried, imaged by exposure to activating radiation, and developed to form a relief image.
It is known that photoresist coating compositions contain particulate and ionic contaminants. For example, it is known that solid gels or insolubles form in photoresists due to dark reactions. In addition, soluble impurities such as moisture, silicone oils, plasticizers, and metal ions may be present from the manufacture of the resist components and from the packaging containers or dispensing tanks. Trapped bubbles from point-of-use filtration or the shaking of a resist bottle prior to dispensing can lead to defects in resist coatings. In Class 100 clean rooms, airborne particulate counts amount to 3 particles per liter of density of 2. By comparison, liquids contain about 100,000 particles per liter. A particle count of 100,000 per liter seems high, but if translated into a solid of 0.6.mu. in size (entity of 2), this is equivalent to 10 parts per million (ppm). A level of 10 ppm amounts to the deposition of 10 mg per liter. Since liquids are heavily contaminated compared to clean room air, effective contaminant removal is essential to the manufacture of such devices.
Filtration of resist liquids has progressed and manufacturers of resist now supply resist materials filtered through 0.04 .mu.M diameter absolute filters. Other methods for removal of particulates such as gels include ultracentrifugation, electrostatic treatment of the resist, and depth filtration. Particulate removal from photoresists such as by filtration is not only practiced by the resist manufacturer, but is often practiced by the device manufacturer by placement of a filter between the resist dispenser and the device whereby the resist is filtered as it passes from its container onto a wafer. This procedure is known in the art as "point-of-use" filtration. A discussion of point-of-use filtration is presented by Cambria et al in SPIE Vol. 1466, "Advances in Resist Technology and Processing VIII", (1991), pp 670-675 incorporated herein by reference and by Asumi et al in SPIE Vol. 1672, "Advances in Resist Technology and Processing IX", (1992), pp 617-622.
Methods useful for removal of particulates from treatment solutions are not effective for removal of dissolved contaminants from solution such as organic impurities and ionic species. These contaminants can be at least as damaging to an integrated circuit as particulate contamination.
The removal of dissolved cationic and anionic contaminants from treatment solutions used to manufacture integrated circuits is known in the art. For example, one such method is disclosed in International Publication No. WO 93/12152, incorporated herein by reference, which is directed to removing metal ions such as sodium and iron from novolak resins during manufacture. The process comprises cation exchange treatment whereby a cation exchange resin is first washed with a mineral acid solution to reduce the level of total sodium and iron ions within the exchange resin to preferably less than 100 ppb, passing a formaldehyde reactant through the so treated cation exchange resin to decrease the sodium and iron ion content to less than 40 ppb, passing a phenolic compound through the cation exchange resin to decrease its sodium and iron ion content to less than 30 ppb, and then condensing the so treated phenolic compound with formaldehyde in the presence of an acid catalyst to form the resin.
A method for removal of ionic metals and non-metals from a photoresist is disclosed in published Japanese Patent Application No. 1228560, published Sep. 12, 1989, incorporated herein by reference. In accordance with the procedures of this application, a photosensitive resin is passed through a mixed bed of a cation exchange resin and an anion exchange resin to simultaneously remove cation and anionic species from the photoresist solution.
In copending U.S. patent application Ser. No. 08/128,994, filed Sep. 30, 1993, assigned to the same assignee as the subject application and incorporated herein by reference, an improved process is disclosed for removing metallic cations from organic solutions using modified cation exchange resins. In accordance with the process of the invention disclosed therein, the cation exchange resin is modified by replacement of the acid protons on the cation exchange groups with essentially neutral groups such as ammonium or amine groups. Thereafter, an organic solution containing acid labile components may be treated with the modified cation exchange resin to remove metal ions without the formation of undesired by-products caused by attack of acid protons on acid labile groups.
In copending U.S. patent application Ser. No. 08/143,489, filed Oct. 27, 1993, assigned to the same assignee as the subject application and incorporated herein by reference, an improved process is disclosed for removing non-metallic anions from organic solutions using a modified anion exchange resin. In accordance with the process of the invention disclosed therein, the anion exchange resin is modified by replacement of the strongly basic groups on the anion exchange groups with essentially neutral or slightly acid groups such as acetate or citrate. Thereafter, an organic solution containing base labile components may be treated with the modified anion exchange resin to remove non-metallic anions without the formation of undesired by-products caused by attack of the strong base on abase labile groups.
A method for removal of organic contaminants together with chelated multivalent metal ions from organic solutions is disclosed in copending U.S. patent application Ser. No. 08/139,923, filed Oct. 20, 1993, assigned to the same assignee as the subject application and incorporated herein by reference. In accordance with the procedures of said copending application, multivalent metal ions are first chelated with a chelating agent for the metal ions, preferably a chelating agent that is an organic impurity in the photoresist formulation such as an azo or diazo dye. The solution of the photoresist containing the chelated, multivalent metal ion is then contacted with a pure activated carbon which selectively removes organic contaminants from the solution of the photoresist including the organic contaminants chelated with the multivalent metal ions.
An additional method for removal of dissolved contaminants from a photoresist containing both acid and base labile materials is disclosed in U.S. patent application Ser. No. 08/149,041 , filed concurrently herewith, assigned to the same assignee as the subject application and incorporated herein by reference. In accordance with the invention of said application, a mixed bed of cation and anion exchange resins, or an ion exchange resin having both anion and cation exchange groups on its backbone, is treated with an ammonium salt of a weak acid to displace both the strong anions and cations to thereby avoid attack on acid and base labile materials.
The above methods for purification utilized by a photoresist manufacture are capable of decreasing the concentration of contaminants from organic solution to acceptable levels. However, it has been found that following manufacture and packaging of the ultrapure materials, contaminants are often found in previously purified materials. The source of these contaminants may be the containers for the composition, pumps and tubing used to convey the composition from its container to a wafer surface and other sources found at the device manufacturer's facility.